The present invention relates to precision macroscopic optical mirrors for scanning applications, in general, and more particularly to a Silicon wafer based macroscopic mirror suitable for meeting the rigid specifications for wide angle scanning mirror systems.
Conventional precision macroscopic mirrors for scanning applications generally include a substrate material which has one surface highly polished and coated with a reflective medium. In operation, these precision mirrors are typically mounted to a drive mechanism, such as a resonant scanner, for example, for scanning of an optical beam incident upon the reflective medium. Common substrate materials include BK-7, Pyrex, Zerodur, Aluminum and the like, for example. (BK-7 and Zerodur are trademarks or tradenames of Schott Corporation and Pyrex is a trademark or tradename of Corning Corporation).
Desired specifications for a wide angle scanning application of a modern mirror system may include a large aperture macroscopic mirror having a reflective surface on the order of 50.8 mm by 71.8 mm, operated at a scanning frequency which may be approximately 100 Hz to direct a large diameter laser beam on the order of at least 20 mm over an optical scan angle of 30° to 40° peak to peak, for example. Preferably, the optical scan is a raster scan type. In some applications, the mirror may perform a dual function as a receiving optic for receiving the backscatterings of the laser beam from objects in its path. For this purpose, it is desirable that the mirror surface have a nearly 100% fill factor, i.e. ratio of usable area to total area of receiving surface.
To meet these specifications, the mirror substrates should be lightweight and highly resistant to deflection. This resistance is measured by a specific stiffness value (E/ρ), i.e. Young's Modulus divided by mass density (rho). The required stiffness for the conventional substrates noted above is generally achieved with a high thickness to diameter ratio, but this results in undesirable added mass to the mirror. This added mass has an impact on the scan drive requirements of the drive mechanism, the attainable resonant frequency of the mirror, and the overall optical system architecture in both cost and weight.
In some applications, such a wide angle mirror system is intended for use in an aircraft environment wherein the precision macroscopic mirror thereof would encounter substantial temperature extremes during flight profiles. In these applications, it is desirable to have a mirror that is highly resistant to thermal distortion over a wide operating temperature range. In addition, the reflective surface of these precision mirrors may need to take upon a variety of profiles based on the particular application. The current substrate materials noted above are not easily and/or inexpensively shaped for different applications. For example, to reshape a standard glass substrate from a simple circular shape to an elliptical shape, a basic solid glass cylinder is often sectioned at a forty-five degree (45°) angle which causes the sides to be beveled. This can make it more difficult to effectively balance and mount the elliptically shaped mirror to the drive mechanism since the center of mass of the mirror will not coincide with the desired rotational center of the reflective surface.
Accordingly, for wide angle scanning applications, it is desirable to have a precision macroscopic mirror with a substrate material that is readily available and can be easily and inexpensively altered in shape. It is also desirable to have such substrate material include high strength and low weight characteristics and offer good resistance to thermal distortion over wide operating temperature ranges. Such properties in a mirror can minimize the scan drive requirements and overall optical system architecture, rendering a lighter weight and more cost-effective optical system than what currently exists.
It is known to use Silicon as a substrate material for microscopic optical mirror applications, such as fiber optic telecommunication switching, for example. An example of the use of Silicon as a mirror substrate for microscopic optical applications is disclosed in the U.S. Pat. No. 6,379,510, entitled “Method of Making A Low Voltage Micro-Mirror Array Light Beam Switch”, filed on Nov. 16, 2000 and issued Apr. 30, 2002 to Kane et al. (hereinafter “Kane”). Kane describes a light beam optical switch for fiber optic telecommunications wherein a micro-mirror can be used to reflect light from a fiber entering the switch junction to a targeted fiber for continuation of the light signal to its appropriate destination (column 2, line 3). Kane's optical micro-mirror switch is fabricated using micro-electro-mechanical systems(MEMs) techniques. Light into and out of a fiber would likely be on the order of 1 mm diameter maximum (may be much smaller) during the time of contact with the mirror. Presumably, the individual micro-mirror of Kane would be slightly larger in size than the directed beam. The U.S. Pat. No. 5,903,380 Motamedi, et al (referenced by Kane) describes a MEMs optical resonator wherein all components fit into a common TO-8 semiconductor housing which is considered quite small. Other patents directed to MEMs fabricated micro-mirror systems refer to a large micro-mirror size as being in the range of 200 um×200 um to 2 mm×2 mm. As a result, MEMs micro-mirror devices are typically developed in a mirror array format.
Kane discusses using an array of micro mirrors tightly packed together (see column 11, line 50). As an optical receive mirror, the larger size effected by the mirror array provides a much greater collection area for better collection efficiency. However, the critical fill factor of such an array of separate MEMS fabricated micro-mirrors pursuant to Kane, for example, would be substantially less than 100%. There would still be considerable loss of collection light due to reduced mirror coverage across the area covered by the array, specifically due to spaces between the micro-mirrors (even if these mirrors are hexagonal and nested as per Kane column 11, line 50) and area taken up by micro-mirror actuation components.
Kane further describes (see Column 4, line 30) an optical network switching frequency of speeds approaching 1 kHz. Often, MEMs micro-mirror devices that operate at these frequencies can attain a deflection angle of no more than a few degrees. The physical construction of the MEMs actuator, designed to meet other criteria, is often a limiting factor to the attainable scan angle. Typically, there is a design trade-off between scan frequency, scan angle and mirror size. For example, in the paper “Electrostatic micro torsion mirrors for an optical switch matrix” by Toshiyoshi et al., Microelectromechanical Systems, 1996, pp. 231-237 which was referenced in the U.S. Pat. No. 6,201,629, a scanner micro-mirror of dimensions 400 um×400 um×30 um, considered to be a relatively large mirror on the micro scale, could deflect over an angle of about 30°, but was limited to a scanning frequency of 75 Hz due to thinness requirements in the torsion members. This scanning frequency is considerably less than that specified for the desired system, principally due to shortfalls of the support structure.
In addition, MEMs micro-mirror devices which utilize silicon as their mirror substrate typically incorporate their actuation system components directly into the same substrate through more extensive and complex microfabrication techniques, i.e. photolithography, micro-etching, thin film deposition, for example. Kane (at Column 3, line 21 and Column 10, line 37) discloses the use of thin films and micro-fabrication methods with a design (see Column 3, line 44) wherein the hinges and supporting structure are located underneath the mirror and actuators are fabricated of the same material. In general, Kane's process (see Column 7, line 41) forms an extremely thin optical switch, whereby the components are essentially coplanar. Support components and a piezoelectric layer are deposited on top of the base layer in separate phases. Specific areas are then etched away to leave the actuators and support structure and finally a mirror surface area. This surface is then deposited with a reflective coating. A separate integrated drive circuit (chip) may be bonded to the optical switch to provide a functional unit. Specifically, Kane's process (see Column 10, line 37) involves 10 major steps (with other smaller steps imbedded) for micro-fabrication that start with a small silicon wafer substrate and end up with a compact array of micro-scanners that can be packaged with its own CMOS-compatible driving circuit creating a very small form factor (see Column 11, line 45).
Ostensibly, silicon based micro-mirror devices made by micro-fabrication techniques, like MEMs, for example, will not meet the desired specifications for a wide angle scanning application of macroscopic mirror system. Accordingly, the present invention overcomes the drawbacks of such silicon based micro-mirror devices and offers a macroscopic mirror suitable for use in a mirror system which meets the desired specifications for wide angle scanning applications.